For example, JP-A-2002-131068 discloses a map display device configured to indicate a character string of only one of two names higher in the priority and configured to prohibit indication of a character string of the other of the two names lower in the priority when the character strings of the two names overlaps one another in the map. Further, the map display device is configured to re-indicate the character string, which is prohibited from indication, when the character string, which is prohibited from indication, is not overlapped any longer. In the JP-A-2002-131068, the two names having the character strings overlaps one another are names of different two features. It is noted that, according to an inventor's study, indication of two character strings of names of, for example, the same features may occur from now on. The reason is that, as a new method for updating of map data, an area update operation may be in practice to update only a part of a geographic range in map data. When such an area update operation is performed, redundant indication of the same character strings may occur, as shown below.
FIGS. 4A, 4B show a first example of redundant characters. In the example of FIG. 4A, in the map data before being updated, a feature 24 exists in an area 21 close to a boundary 23 between the area 21 and an area 22, and a character string 25 of a company name of “SUNDAY” is indicated close to the company 24. In the present condition, it is assumed that the area 22 is updated without updating of the area 21 by the area update operation. By the present area update operation, while map data of the area 22 is updated to a new version of the map data, the previous version of the map data of the area 21 remains. In the present condition, it is assumed that position information on the feature 24 was inaccurate or wrong in the previous version of map, and therefore, the position information on the feature 24 is corrected to accurate information in the new version. Consequently, it is assumed that even though the feature 24 belonged to the area 21 in the previous version of the map data, the same feature 24′ becomes to belong to the area 22 in the new version of the map data. In this case, when the area 22 is updated, and when the area 21 is not updated, the same two features 24 and 24 may exist closely to each other near the area boundary 23 in the map data. As shown in FIG. 4B, when a map indicating operation is performed according to the map data, the name character strings 25 and 25′ of the same name of “SUNDAY” may overlap one another and may be indicated. Alternatively, as shown in FIG. 4C, the name character strings 25 and 25′ of the same name of “SUNDAY” may be closely indicated, even thOugh not being overlapped one another.
Further, FIGS. 5A, 5B show a second example of the redundant characters. In the example of FIG. 5A, similarly to the example of FIG. 4A, before updating of the map, one street (specifically, National Route Meishi, National Route 23) 26 extends through both areas 21 and 22 over the boundary 23 between the area 21 and the area 22. In this case, it is conceived that a link sequence record, which includes a link group 26a of the street 26 in the area 21, is stored as a link sequence record belonging to the area 21, and a link sequence record, which includes a link group 26b of the street 26 in the area 22, is stored as a link sequence record belonging to the area 22. In this case, since the link group 26a and the link group 26b are originally a part of the same street 26, the street name corresponding to the link sequence record of the link group 26a is the same as the street name corresponding to the link sequence record of the link group 26b. Therefore, two street names corresponding to the two link sequence records specifying the same street are indicated. Consequently, as shown in FIG. 5A, the same multiple street names 27 and 27′ may be indicated on one display screen. Thus, the appearance of the map may be impaired and complicated.
In view of the present problem, when a map data manufacturer creates the same version of map data over multiple areas using a computer, it is conceived to beforehand configure map data not to redundantly indicate multiple names of one street in one screen of an image display unit. For example, as shown in FIG. 5A, the map data may be configured to indicate only the name 27 among the same street names 27 and 27′ and not to indicate the name 27′. However, as a result of the area update operation, when the area 22 is updated and when the area 21 is not updated, configuration of indication/non-indication of the names may not be appropriately performed in different versions of map data. Therefore, as shown in FIG. 5B, as a result of the area update operation, the same street names 27 and 27′ are indicated in the adjacent areas. Consequently, the same names of the one street overlapped one another may be indicated on one screen of an image display unit.